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STATES OF MATTER
CH 11.4-7 CH 11.1-3 CH 5 The internet?
Phase Change-The transformation from one phase to another upon the addition or release of energy. Temperature does not change during a phase change.
4
Phase Changes
Outline• Liquid ↔ Gas (p 496-501)
• Solid ↔ Liquid (p 501-503)
• Solid ↔ Gas (p 504-505)
MostOrder
LeastOrder
Liquid ↔ GasKinetic energy of molecules.
“slow” molecules
“fast” moleculesSome molecules are fast enough to escape intermolecular forces.
Liquid ↔ Gas
At time = 0 At time > 0 At time = ∞
Liquid
# of molecules in =# of molecules out
At equilibrium!
rate of evaporation = rate of condensation
Liquid ↔ GasMolecular motion causes force.
Force creates pressure.We can measure the pressure.
• Vapor pressure (VP)- is the pressure exerted by a vapor in equilibrium with its condensed phases (solid or liquid) at a given temperature in a closed system.
• # of molecules in =# of molecules out
• rate of evaporation =rate of condensation
• At a given temperature:
Weaker intermolecular forces = more molecules out =
higher vapor pressure
Vapor pressure is temperature dependent!
8
Vapor pressure is temperature dependent!
Liquid ↔ Gas
Higher temperature (T) =
higher average kinetic energy =
more molecules escaping solution =
higher vapor pressure (P)
Lower T =
Lower P
Higher T =
Higher P
Ch 11.8Page 496
9
Relationship Between T and PThe experiment-
•Close container with pressure gauge.•Filled with liquid.•Increase temperature and measure pressure.•Graph pressure vs. temperature.
Hot Plate
Ch 11.8Page 497
Each point on the line is the boiling point at that pressure.
10
Relationship Between T and P
y = m • x + b
ln P =DHvap
T+ C
Clausius-Clapeyron EquationR
•-1
P = vapor pressure (atm)
T = temperature (K)
R = gas constant (8.314 J/K•mol)
DHvap = molar heat of vaporization (J/mol)
Ch 11.8Page 498
ln P =DHvap
T+ C
R•-
1
Clausius-Clapeyron Equation
P = vapor pressure (atm)
T = temperature (K)
R = gas constant (8.314 J/K•mol)
DHvap = molar heat of vaporization (J/mol)
Relationship Between T and P
Molar heat of vaporization (DHvap) is the energy required to vaporize 1 mole of a liquid at its boiling point.
Dependent on the substance and its intermolecular forces.more intermolecular force = larger DHvap
Independent of temperature.
12
Alternate Forms of the Clausius-Clapeyron Equation
If we know DHvap, P and T at one temperature we can predict P at another T.
or
Relationship Between T and P
Slope = change in y/the change in x
Ch 11.8Page 499
or T (bp) at another P.
11.7
Diethyl ether is a volatile, highly flammable organic liquid that is used mainly as a solvent.
The vapor pressure of diethyl ether is 401 mmHg at 18°C and the Hvap = 26.0 kJ/mol. Calculate its vapor pressure at 32°C.
P = vapor pressure (atm)
T = temperature (K)
R = gas constant (8.314 J/K•mol)
DHvap = molar heat of vaporization (J/mol)
11.7
Solution Table 11.6 tells us that Hvap = 26.0 kJ/mol. The data are
From Equation (11.5) we have
Taking the antilog of both sides (see Appendix 4), we obtain
P2 = 656 mmHg
Check We expect the vapor pressure to be greater at the higher temperature. Therefore, the answer is reasonable.
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Closed vs. Open Systems
Closed System Open System
has a Boiling Point
at equilibrium not at equilibriumliquid molecules lost
Boiling Point
vapor pressure < external pressure
Vapor pressure
External pressure
vapor pressure > external pressure
Vapor pressure
External pressure
Lower Temp Higher Temp
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Vapor Pressure and Boiling Point• The boiling point is the temperature at which the liquid’s vapor
pressure is equal to the applied external pressure.
• The normal boiling point is the boiling point when the pressure is exactly 1 atm.
• Decreasing the applied external pressure results in a lower boiling point.
Solid ↔ Liquid
Molar heat of vaporization (DHvap) is the energy required to vaporize 1 mole of liquid to gas.
Molar heat of fusion (DHfus) is the energy required to melt 1 mole of solid.
Ch 11.8Page 501
Solid ↔ Gas
• Sublimation is the direct transition from the solid to the vapor state “bypassing” the liquid phase
• During sublimation the solid and gas phases are in equilibrium
Iodine (I2) Ice Cubes
Ch 11.8Page 504
Solid ↔ Gas
Molar heat of vaporization (DHvap) is the energy required to vaporize 1 mole of liquid to gas.
Molar heat of fusion (DHfus) is the energy required to melt 1 mole of solid.
Molar heat of sublimation (DHsub) is the energy required to sublime 1 mole of a solid.
DHsub = DHfus + DHvap
( Hess’s Law)
• Temperature vs. Heat added curve.
• The temperature of the substance does not rise during the phase change.
• The heat added to the system at the melting and boiling points goes into breaking intermolecular forces.
Phase Change and Heating Curve
Ch 11.8Page 503
Phase DiagramA phase diagram summarizes the conditions at which a substance exists as a solid, liquid, or gas and the places where equilibria exist between phases.
Ch 11.9Page 505
Phase Diagram
Important Notes:
Each line (AC, AB, and AD): two phases are in equilibrium.
B) The critical point: above this critical temperature and critical pressure the liquid and vapor are indistinguishable from each other.
Below A: the point below which the substance cannot exist as a liquid .
A) The triple point: the point at which all three states are in equilibrium.
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Phase Diagram of WaterNormal melting and boiling points.
Solid to liquid with applied pressure.
Triple Point (0.006 atm and 0.01 °C)